The present invention relates to a method of mutually adjusting single-colour images on a multicolour rotary printing press.
Known multicolour rotary printing presses comprise a number of printing units, one for each colour, arranged side by side and fed one after the other with a continuous strip of paper fed along a given path by a number of printing cylinders, one for each printing unit, each of which provides for printing the paper strip with a given colour ink.
The cylinders on all the printing units are usually powered by the same drive via a transmission for synchronously rotating all the printing cylinders.
On known rotary printing presses of the aforementioned type, the printing cylinders are normally connected in fixed manner to the transmission, and, for achieving the required multicolour image, the single-colour images produced by the respective cylinders are mutually adjusted by means of a guide roller on each printing unit, about which the strip for printing is fed. On each printing unit, the adjusting roller is normally mounted between two fixed guide rollers, so as to define a bend in the path of the paper strip, and is movable in relation to the fixed rollers and across the strip, so as to adjust the length of the bend and, with the press running, adjust the single-colour image produced on the printing unit with a corresponding image printed on the upstream unit.
The above method of mutually adjusting or timing the single-colour images presents a number of drawbacks, not only due to the relatively complex mechanical design of the devices regulating the position of the movable adjusting rollers, but also on account of the length of the path of the paper strip and the time required for making the adjustment.
In connection with the above, it should be pointed out that, on a rotary printing press, each bend for adjusting the strip increases its path by about one meter, thus increasing the risk of the strip tearing during printing and the time required for resetting the torn strip. What is more, as adjustment is made with the press running, and transverse displacement of the adjusting rollers is necessarily slow to prevent inducing undue transient stress on the strip, the amount of printed matter rejected during adjustment is normally considerable.